... 500 Singaporeans have been tested positive for HIV (up to late- 1996), however, this is probably only the tip of the iceberg as there are many others who are infected but do not know that they are. In the 1980s, the majority of infections in Singapore were homosexual or bisexual men; but now there are many infections seen among heterosexual men and women. Heterosexual sex is now the main method of transmission in Singapore and over 75% of infections throughout the world are acquired through heterosexual intercourse. Not everybody who has sex with an infected person will get infected. However, the risks increase sharply in the presence of other Sexually Transm ...

... butane or Freon. The gas is partly liquefied by the pressure in the can, but there is a layer of free gas above the liquid. As the can empties liquefied gas vaporizes to fill the space. The valve is normal held shut by the pressure in the can, and by the coil spring directly below the valve stem. When the push button is pressed, it forces the valve stem down in its housing, uncovering a small a small hole which leads up through the stem to the nozzle in the button. This allows the product to be forced up the dip tube by the gas pressure in the can. The nozzle is shaped to give a spray or a continuous stream. To produce a fine mist, a propellant is used which mixes ...

... of organic fibers held together by an organic cement which gives bone its tenacity, elasticity and its resilience. It also has an inorganic component located in the cement between the fibers consisting of calcium phosphate [85%]; Calcium carbonate [10%] ; others [5%] which give it the hardness and rigidity. Other than providing the rigid infrastructure, it protects vital organs like the brain], serves as a complex lever system, acts as a storage area for calcium which is vital for human metabolism, houses the bone marrow within its mid cavity and to top it all it is capable of changing its architecture and mass in response to outside and inner stress. It is ...

... components of acid rain. Its mainly caused by exhaust fumes. Like sulphur dioxide, the nitrogen oxides rise into the atmosphere and are oxidized in clouds to form nitric acid. One of the most serious impacts of acid precipitation is on forests and soils. A lot of damages are being done when the sulphuric acid falls onto the earth as rain. All the nutrients in the soils are washed away making it hard for some plants to grow. The presence of nitrogen oxide and nitric oxide also can force trees to grow even though they do not have the necessary nutrients. The trees are sometimes forced to grow during late autumn when it is actually the time for them to prepar ...

... lineage": This is a sequence of similar genera or families, linking an older to a very different younger group. Each step in the sequence consists of some fossils that represent certain genus or family, and the whole sequence often covers a span of tens of millions of years. A lineage like this shows obvious intermediates for every major structural change, and the fossils occur roughly (but often not exactly) in the expected order. However, usually there are still gaps between each of the groups. Sometimes the individual specimens are not thought to be directly ancestral to the next-youngest fossils (e.g. they may be "cousins"" or "uncles" rather th ...

... Insecticides are used by farmers who want to exterminate some of the many harmful insects that can carry disease. Insecticides are very widely used, not only on farms, but in residential areas in the form of products like Raid, and ant traps. are dangerous to our environment, but in most cases they help more than they hurt. For example, it is estimated that the food supply would reduce by forty to fifty percent without the use of on our crops. The FDA, EPA, and the Department of Agriculture try to ensure that the use of is necessary to the extent in which they actually are used by performing studies and proposing bills and laws to Congress. There is ...

... and 1950's. The needs of the military at the time required that a system be created that could aid in the aiming of ballistic military equipment. Today’s modern computers are undoubtedly the result of the United States military’s early developments and technological advancements in computing. Because of the military’s need for the computer earlier this century, great advancements have been, and will continue to be made in the way of computers and other technologies. Truly the first computers, or tools made with the intention of being used for calculating, were made many centuries ago. Throughout history, people have used a diverse array of devices to help them do bo ...

... star has the potential to become a black hole. The definitions of black boles are somewhat sceptical. Generally, a black hole is an area of super-concentrated mass. So concentrated, that no object can escape its gravitational pull. In other words, once you get caught by it’s graviational pull, you aren’t getting out again. The velocity you need to break away from a graviational pull is called the "escape velocity". Roughly, earth’s escape velocity is about 25,000 M.P.H. (11.2 kilometers/second). Earth’s mass is nothing compared to the mass of a star that has the potential to become a black hole. A black hole has so much mass in such a small area, that its escape ...

... As a breakthrough technology today, computers have replaced pens and papers. Many businesses are now relying on computers because their statistics and calculations are more accurate than manual calculations. These machines are enabling us to perform many different tasks in less time and making our lives easier. Computers in telecommunications have allowed us to do many things from the comfort of our homes that could never be done before; such as talking with distant friends and relatives, buying merchandise, doing research, and a steadily increasing number of other things. At nearly any time of day we can buy something with a credit card, or fill out ...

... called a "period" of elements. Then they repeat, through the next period. To express this, Mendeleev stated the periodic law: The properties of the elements are periodic functions of the atomic weights. provides an easy way to show the division of the elements into acid-forming and base-forming types. Those on the left are base-forming; those on the right are acid-forming. Those in between can be either. They have so-called amphoteric oxides and hydroxides that can act like acids or bases. 's arrangement also divides the elements into metallic and nonmetallic kinds. A distinction is usually made between most pure metals and nonmetals according to physica ...